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Ancient boundary of the Roman Empire, Europe’s second longest river flows from the Black Forest of Germany through the Austro-Hungarian capitals of Vienna and Budapest to the capital of their enemy Serbia, then forms the border of Bulgaria and Romania on its way to the Black Sea. In peacetime, it is a major highway of regional food and trade systems; since midnight on the morning of July 29th, 1914, when the SMS Bodrog announced the opening of hostilities by firing on Belgrade, the Danube river has instead been a fault-line of the conflict.

During the first five months of war, the Imperial and Royal Army crossed Serbia’s riverine boundaries three times, with each invasion repulsed despite overrunning Belgrade in December. But now the Hapsburgs are back, and this time they have allies. Opening a preparatory bombardment tonight, the Austrian Third Army begins to fight its way across the Danube and Sava rivers at Belgrade while the German Eleventh Army crosses downstream at Ram. Pontoons are floated out onto the water under fire, and progress is hard-won at great cost, but by tomorrow afternoon they have established a firm bridgehead onto Serbian soil.

Serbian lookouts on the larger of two so-called ‘Gypsy Islands’ in the Sava, lost to the 43rd Reserve Infantry Division today

Which is not to say that the Serbs have not spent the last nine months resting on their laurels. The Central Powers are attacking into the teeth of prepared defenses, but the Serbian Army is already stretched to the limit in material and personnel. Tomorrow, as street fighting begins in Belgrade, which is already afire from shelling despite the heavy pall of rain, Field Marshall August von Mackensen sends the rest of his forces onto the opposite bank, whereupon his command outnumbers the Serbian defenders three to two, with a three to one advantage in artillery tubes. As always, Mackensen’s logistical arrangements are superior, crushing the enemy under the weight of his fire. While the infantry presses forward in a flood, a stream of ammunition and supplies follows behind. When high winds damage his pontoon bridges over the Danube on the tenth day of operations, his commandeered steamers redouble their work to take up the slack. The muddy bogs that pass for roads become a greater hindrance than the Serbian enemy.

In Nis, the temporary wartime capital of the Serbian Kingdom, Peter I despairs of taking any advantage of his foe’s communications problems. Despite the highest recruiting rate of any power at war, Serbia faces too many foes on too broad a front this time. If he was only fighting the kaisers, he would have plenty of men and guns for a sound defense, but Peter also faces the aggression of a tsar. His chief of staff Radomir Putnik has already deployed as much of his army as possible to defend the Bulgarian frontier.

A watercolor of a river monitor supporting bridging operations near Belgrade. Note how the bridge sections must be floated out and assembled under fire. Temporary bridges are easily-broken by storms and artillery, but also easy to build and repair

Today, the Romanian Army is belatedly fortifying Giurgevo, the city where one of the two Balkan railway routes to Istanbul crosses the Danube into Bulgarian territory. Alarmed by Bulgaria’s evident intentions to attack Serbia, Romania is hardly the only neutral power being destabilized by Sofia’s new alliance with Vienna and Berlin, and today’s mobilization reflects the rapid increase in regional tensions that has occurred in just the last several days. Just forty miles from Giurgevo, Bucharest nevertheless proves unready and unwilling to intervene, judging the risks too high in comparison to the rewards offered by the Entente allies.

And there are delays in Bulgarian mobilization, too, for the nation is less eager for war in 1915 than it was in 1912, while Tsar Ferdinand I never restored his armories or his armies after the defeats of 1913. Informing his new allies of the postponement, Ferdinand’s assault will begin on the fifth day of Mackensen’s operations; official declarations of war will ensue three days later. At that point, the sheer weight of mass descending on Serbia will be too overwhelming to stop.

Austrian field artillery moving through the outskirts of Belgrade tomorrow; note the effects of shelling

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